Magnetic tape, optical disk memory and other mediums, are commonly used recording mediums for recording/reproducing video signals or other signals. Recently, optical disk memories in which handling is simple and operation is excellent, has been used for a variety of tasks.
Different types of optical disk memory are known;
the Read-Only type optical disk memory, where information is recorded during the manufacturing stage of the optical disk memory, such as for instance compact disks, laser video disks, etc.,
the Direct Read After Write type optical disk memory where information can be recorded only once, such as the perforated type memory employing Te-C or other material as a recording layer, and
the Rewritable type optical disk memory where information can be recorded and reproduced as many times as desired, such as the magneto-optical disk, and the like.
Recording on a Read-Only type optical disk memory, for example, is performed as follows. First, the original disk is cut by means of an Ar laser or other laser thereby forming a recording master. Then, recording pits are reproduced from the recording master on a plastic substrate through an injection molding method or other method. Meanwhile, recording and reproduction on DRAW type and Rewritable type optical disk memories are performed through a laser light that is emitted by a semiconductor laser or other member housed in a recording/reproducing device.
The recording area of such an optical disk memory is limited. Therefore, in order to increase the recording capacity, the recording density needs to be increased as much as possible. However, when recording/reproducing video signals, for example the level of the reproduced signals needs to be above a given value in order to ensure reproduced images of a satisfactory quality. In other words, the signal-to-noise ratio (hereinafter, referred to as S/N) of the reproduced images needs to be above a given value. The recording capacity of the optical disk memory is thus determined by the minimal length of the recording bits permitting to obtain reproduced signals of an amplitude above the given value. Here, the length of the recording bits extends along the direction of recording tracks formed on the optical disk memory. Accordingly, in order to increase the recording capacity of the optical disk memory without causing a drop in the level of the reproduced signals, the recorded density needs to be enhanced by reducing the minimal length of the recording bits.
Suppose now that video signals are recorded on the optical disk memory after having gone through an FM process. When the length of the recording bits is smaller than the radius of a light spot (that depends on the wavelength of the laser light emitted by the Ar laser, the semiconductor laser or other laser) formed as a converging beam irradiated on a recording track, the level of the reproduced signals suddenly drops thereby causing the quality of the reproduced images to lower. In conventional devices, the minimal bit length permitted for recording bits to be recorded/reproduced and to have a satisfactory reproduction performance, is approximately equal to 0.5 .mu.m for a Read-Only type optical disk memory, and approximately equal to 1 .mu.m for DRAW type and Rewritable type optical disk memories.
Reducing the pitch of the tracks formed on the optical disk memory might be considered as means for enhancing the recording density. However, if the pitch of the tracks is reduced so as to be approximately equal to the radius of the light spot, crosstalk occurs between adjacent tracks. The smallest track pitch possible is thus approximately equal to 1.6 .mu.m, i.e. the track pitch commonly adopted.
For the above reasons, in conventional devices the recording time of, for example, a 300 mm Read-Only type laser disk that rotates at Constant Angular Velocity (hereinafter referred to as CAV) at a speed of 1800 rpm, is relatively short, especially when video signals that contain a large amount of data are recorded, and is equal to about only 30 minutes. The recording time for a 200 mm laser disk is even shorter and is approximately equal to about 14 minutes with the CAV method, and about 18 minutes with the Constant Linear Velocity (hereinafter referred to as CLV) method.
As described above, with the conventional technology, the recording density cannot be satisfactorily increased by reducing the length of the recording bits or the pitch of the tracks. Conventional technology thus does not offer a long recording time, especially when video signals that contain a large amount of data are recorded.